Zero-voltage Ride-through Research Articles

Zero-Voltage Ride-Through Scheme of PMSG Wind Power System Based on NLESO and GFTSMC

In this paper, a scheme for zero-voltage ride through of a permanent magnet synchronous generator wind power system is proposed. Maintaining the stability of DC-link voltage is the key to realizing zero-voltage ride through. A braking chopper is used to dissipate the active power to restrain the rise of DC-link voltage during a grid fault. However, the braking chopper-caused disturbance to the double closed-loop control of the grid side converter makes the control effect on DC-link voltage worse. Therefore, a robust current feedforward control, based on a nonlinear extended state observer and global fast terminal sliding mode control is proposed. A nonlinear extended state observer estimates the total disturbance in the system and compensates for the control law. Global fast terminal sliding mode control enables the system to reach the sliding mode surface in a finite time, and its control law is continuous without switching terms, thereby eliminating the chattering phenomenon. A nonlinear extended state observer and global fast terminal sliding mode control change the current feedforward control into a nonlinear robust current feedforward control. The control effect is improved, and the use of current transformers is reduced in practical applications, thereby reducing costs. The validity of this scheme has been verified by simulation.

A reduced-order model for representing the reactive power response of a doubly fed wind turbine during ZVRT

Zero-voltage ride through (ZVRT) is the worst-case scenario of low-voltage ride through (LVRT), which indicates the optimal fault-response capability of wind turbines (WTs). The reactive response during ZVRT shows the dynamic performance of WTs during extreme grid faults, which greatly improves the flexibility and robustness of renewable-rich grids. The reactive response of a doubly fed (type-3) WT during ZVRT depends on the control strategy of the converter and the steady-state and transient-state performance of the doubly fed induction generator (DFIG). This paper proposes a reduced-order model for representing the reactive response of a type-3 WT during ZVRT including the control strategy of the converter and the equivalent electric circuit of the DFIG, which simplifies some complicated equations but is practical for modeling a renewable-rich power system in most cases. Some simplified expressions between the reactive energy and a single DFIG parameter are presented, which could satisfy the demands of designing a DFIG. Finally, the method of model consistency verification, the influence analysis of DFIG parameters on the reactive response, the reasonable ranges of DFIG parameters and the fault-response performance of the WT in the theoretically longest ZVRT period are also obtained for reference.

Research on Zero-Voltage Ride Through Control Strategy of Doubly Fed Wind Turbine

With the rapid increase in the proportion of the installed wind power capacity in the total grid capacity, the state has put forward higher and higher requirements for wind power integration into the grid, among which the most difficult requirement is the zero-voltage ride through (ZVRT) capability of the wind turbine. When the voltage drops deeply, a series of transient processes, such as serious overvoltage, overcurrent, or speed rise, will occur in the motor, which will seriously endanger the safe operation of the wind turbine itself and its control system, and cause large-scale off-grid accident of wind generator. Therefore, it is of great significance to improve the uninterrupted operation ability of the wind turbine. Doubly fed induction generator (DFIG) can achieve the best wind energy tracking control in a wide range of wind speed and has the advantage of flexible power regulation. It is widely used at present, but it is sensitive to the grid voltage. In the current study, the DFIG is taken as the research object. The transient process of the DFIG during a fault is analyzed in detail. The mechanism of the rotor overcurrent and DC bus overvoltage of the DFIG during fault is studied. Additionally, the simulation model is built in DIgSILENT. The active crowbar hardware protection circuit is put into the rotor side of the wind turbine, and the extended state observer and terminal sliding mode control are added to the grid side converter control. Through the cooperative control technology, the rotor overcurrent and DC bus overvoltage can be suppressed to realize the zero-voltage ride-through of the doubly fed wind turbine, and ensure the safe and stable operation of the wind farm. Finally, the simulation results are presented to verify the theoretical analysis and the proposed control strategy.

Analysis, simulation and testing of the ZVRT capability of a wind turbine with a doubly fed induction generator

Zero-voltage ride through (ZVRT) is the extreme case of a low-voltage ride through (LVRT), which represents the optimal grid-connection capability of a wind turbine (WT). Enforcing ZVRT will improve the dynamic performance of a WT under the extreme conditions, and therefore significantly enhance the resiliency of a renewable-rich power grid. Compared with LVRT, there are few research on ZVRT with no field validation been conducted. The feasibility of the ZVRT requirement, especially the 400-ms-level fault-time condition, is not clear yet. The first system-level study including the control strategy, transient analysis, hardware enhancement strategy, WT modelling and field test on the ZVRT capability of a Type-3 WT is presented in this study. Particularly, to meet the challenging technical requirements, the authors proposed a hardware enhancement strategy, which accommodates with the widely-used LVRT control approach. The holistic strategy is tested under the extreme grid-connection condition by both simulation and a costly field test. The field test in the 400-ms-level fault-time condition based on an operating WT in a weak power system directly verifies the feasibility and effectiveness of ZVRT for a Type-3 WT. Furthermore, the critical time window and the key parameter ranges during ZVRT, which significantly affect ZVRT performance, are obtained.

A Control Scheme with the Variable-Speed Pitch System for Wind Turbines during a Zero-Voltage Ride Through

Zero-voltage ride through (ZVRT) is the extreme case of low-voltage ride through (LVRT), which represents the optimal grid-connection capability of wind turbines (WTs). Enforcing ZVRT will improve the dynamic performance of WTs and therefore significantly enhance the resiliency of renewable-rich grids. A control scheme that includes a pitch system is an essential control aspect of WTs riding through voltage dips; however, the existing control scheme with a pitch system for LVRT cannot distinguish between a ZVRT status and a power-loss condition, and, consequently, does not meet the ZVRT requirements. A system-level control scheme with a pitch system for ZVRT that includes pitch system modeling, control logic, control circuits, and overspeed protection control (OPC) is proposed in this paper for the first time in ZVRT research. Additionally, the field data are shared, a fault analysis of an overspeed accident caused by a voltage dip that describes the operating status at the WT-collapse moment is presented, and some existing WT design flaws are revealed and corrected by the fault analysis. Finally, the pitching performance during a ZVRT, which significantly affects the ZVRT performance of the WT, is obtained from laboratory and field tests. The results validate the effectiveness of the proposed holistic control scheme.

An Improved Dynamic Voltage Restorer Model for Ensuring Fault Ride-Through Capability of DFIG-based Wind Turbine Systems

Renewable energy sources (RES) are being integrated to electrical grid to complement the conventional sources in meeting up with global electrical energy demand. Among the RES, Wind Energy Conversion Systems (WECS) have gained global electricity market competitiveness especially the Doubly Fed Induction Generator (DFIG)-based Wind Turbines (WTs) because of flexible regulation of active and reactive power, higher power quality, variable speed operation, four quadrant converter operation and better dynamic performance. Grid connected DFIG-based WTs are prone to disturbances due to faults in the network which made the utilization of the power generated a major concern. The grid code requirement for integrating the DFIGs to grid specified that they must remain connected and support the grid stability during grid disturbances of up to 1500milliseconds. The ability of the DFIG WT system to uphold to the grid codes requirement is termed the Fault Ride – Through (FRT). This paper presented a 1.5MW grid connected DFIG-based WT model with a Dynamic Voltage Restorer (DVR) for FRT capability enhancement. The design and simulation were performed in MATLAB/Simulink software. The test system was subjected to disturbances leading to Low Voltage Ride – Through (LVRT), Zero Voltage Ride – Through (ZVRT) and High Voltage Ride – Through (HVRT) considering three – phase balanced fault and single line to ground fault. The performance of improved model of DVR shows enhancement over conventional DVR in terms of voltage compensation and fault current mitigation.

A review of fault ride through of PV and wind renewable energies in grid codes

This paper presents a comprehensive review of fault ride through (FRT) in the grid code of 38 selected countries with an emphasis on renewable energy (REN) sources–related rules. Grid codes are the rules legislated usually by the transmission system operators (TSOs) to determine the grid integration requirements of electrical power generators. Each country establishes its grid code for satisfying the minimum required technical criteria and revises it frequently to cope with new modifications of the utility. Growing the penetration of REN sources have influenced many operational aspects of the power system such as protection, power quality, reliability, and stability. Thereupon, regulations must ensure the power system's secure and controllable operation of REN sources. FRT is one of the main parts of the grid code, and its characteristics affect the performance and rating of power system apparatus. FRT defines the performance of electric power generators during and in postfault conditions. FRT of solar photovoltaic (PV) and wind turbines (WTs) as the main REN sources of energy has great importance in the grid codes. In this paper, a comparison of FRTs in the grid code of 38 countries, including low-voltage ride through (LVRT), zero-voltage ride through (ZVRT), and high-voltage ride through (HVRT) are provided and surveyed.

ZVRT Capability of Single-Phase Grid-Connected Inverter With High-Speed Gate-Block and Minimized <italic> LCL</italic> Filter Design

This paper proposes a zero-voltage ride-through (ZVRT) method and an LCL filter design method to meet the fault ride-through (FRT) requirements for a single-phase grid-connected inverter with a minimized LCL filter. A high inverter output current overshoot occurs at a grid voltage sag and a grid voltage recovery, when a low-inductance LCL filter design is employed in order to minimize the filter volume. In order to suppress the current overshoot, a high-speed gate-block operation and the design method of the minimized LCL filter are proposed. In particular, the filter design considers the delay time in the gate-block operation in order to clarify the reduction limitation of the inductance in the LCL filter. As a result, the maximum inverter output current overshoot is suppressed to 146% by the high-speed gate-block operation and the designed LCL filter, where the impedances of an interconnected inductor and a filter inductor are 1.0% and 0.78% of the inverter normalized impedance, respectively. Therefore, it is possible to meet the FRT requirements with the proposed ZVRT method even with the minimized LCL filter.

Photovoltaic Grid Connected Inverter Crossing Control Method Based on Grid Voltage Feed forward Compensation

According to the requirements of relevant national standards, large photovoltaic grid connected inverters need to have zero voltage ride through (ZVRT) capability to prevent low voltage automatic disconnection, thus affecting the normal and stable operation of power system. On the basis of analyzing the ZVRT standard of PV grid connected inverter, the key technologies of inverter realizing ZVRT are discussed in detail, including grid voltage positive and negative sequence separation and phase locking, active and reactive current control of inverter, system control under unbalanced grid voltage and so on. On this basis, it is further proposed that the grid voltage feed forward component phase lead compensation link should be introduced into the system current loop, so as to improve the transient overshoot of grid connected fault. Finally, the correctness and effectiveness of the ZVRT control strategy of the PV grid connected inverter is verified by the experimental results of RTDS and a 500 kW prototype.

Zero-Voltage Ride-Through Capability of Single-Phase Grid-Connected Photovoltaic Systems

Distributed renewable energy systems play an increasing role in today’s energy paradigm. Thus, intensive research activities have been centered on improving the performance of renewable energy systems, including photovoltaic (PV) systems, which should be of multiple-functionality. That is, the PV systems should be more intelligent in the consideration of grid stability, reliability, and fault protection. Therefore, in this paper, the performance of single-phase grid-connected PV systems under an extreme grid fault (i.e., when the grid voltage dips to zero) is explored. It has been revealed that combining a fast and accurate synchronization mechanism with appropriate control strategies for the zero-voltage ride-through (ZVRT) operation is mandatory. Accordingly, the representative synchronization techniques (i.e., the phase-locked loop (PLL) methods) in the ZVRT operation are compared in terms of detection precision and dynamic response. It shows that the second-order generalized integrator (SOGI-PLL) is a promising solution for single-phase systems in the case of fault ride-through. A control strategy by modifying the SOGI-PLL scheme is then introduced to single-phase grid-connected PV systems for ZVRT operation. Simulations are performed to verify the discussions. The results have demonstrated that the proposed method can help single-phase PV systems to temporarily ride through zero-voltage faults with good dynamics.

Zero-Voltage Ride-Through Capability of a Transformerless Back-To-Back System Using Modular Multilevel Cascade Converters for Power Distribution Systems

This paper focuses on the zero-voltage ride-through (ZVRT) capability of a transformerless back-to-back (BTB) system intended for installation on a power distribution system with many distributed power generators. The BTB system consists of two modular multilevel cascade converters based on double-star chopper cells (MMCC-DSCC) and their ac sides are connected to a common ac mains via ac-link inductors. This paper provides an experimental discussion on the DSCC-based BTB system under voltage sags by using a three-phase 200-V 10-kW downscaled system. Experimental results show that the BTB system is equipped with ZVRT capability without causing any overvoltage and/or overcurrent even under the most severe voltage sags.

A Phase-Shifted PWM D-STATCOM Using a Modular Multilevel Cascade Converter (SSBC)—Part II: Zero-Voltage-Ride-Through Capability

This paper provides an experimental discussion on zero-voltage-ride-through (ZVRT) capability of a phase-shifted pulsewidth-modulation distribution static synchronous compensator (D-STATCOM) using the modular multilevel cascade converter based on single-star bridge cells (SSBC). The cluster-balancing control producing a significant effect on the ZVRT capability is modeled and analyzed with focus on either a low-pass filter (LPF) or a moving-average filter (MAF) to attenuate the 100-Hz (double the line frequency) component inherent in each dc capacitor voltage. The cluster-balancing control using the MAF is better in transient performance than that using the LPF. A three-phase downscaled SSBC-based D-STATCOM rated at 150 V and 10 kVA is designed, constructed, and tested to verify the ZVRT capability under the severest single-, two-, and three-phase voltage sags with a voltage depth of 100%. Experimental waveforms show that the STATCOM continues operating stably as if no voltage sag occurred.